Conventional implant catheter delivery systems comprise an inner shaft that extends from a proximal end to a distal end of the system and carries on its distal end a self expanding implant. A longitudinal tubular sheath lies radially outside the implant to retain the implant in the catheter delivery system until the self expanding stent is to be released.
In WO 2006/133959, which is incorporated by reference in its entirety into this application, there is generally shown a catheter device having such a construction. During a medical procedure, the catheter device is pushed distally into a bodily lumen until the implant is disposed at a pre-determined delivery position. To allow the implant to expand at the delivery position, the outer sheath has to be retracted. For this reason the sheath is connected to a pull wire which is pulled by means of a device to apply a pull force to the wire and, thus, to the outer sheath. To connect the wire to the outer sheath, commonly there is used an outer ring and an inner ring, the outer ring radially outside the outer sheath and being crimped or swaged onto the inner ring which is radially inside the outer sheath. The wire can then be connected to the inner ring, while the outer ring is clamped onto the inner ring, fastening both rings and the wire onto the sheath. When the wire is pulled, the sheath is retracted from where it surrounds the implant and the inner shaft, releasing the implant to expand.
Another possibility shown in WO 2006/133959 for fastening the wire relative to the outer sheath is to provide two outer rings outside the sheath, one on each side of an inner ring inside the sheath, to which the pull wire is fastened. The two outer rings are clamped onto the outer sheath, one proximal and one distal to the inner ring, to prevent relative movement between the outer sheath and the inner ring. In this way the outer sheath is connected to the pull wire by clamping it on either side of the inner ring, to move with the inner ring when the pull wire is retracted.
The implants which are to be delivered can be of a variety of lengths, and some may be quite long, for example up to 120 mm in length and 10 mm in diameter. With a longer implant, due to the higher friction forces between the greater contact surface area of the longer implant and the outer sheath, the retracting force needed to retract the outer sheath will necessarily increase with the length. This is particularly noticeable for covered stent grafts, where the friction with the outer sheath is large, as compared with an uncovered stent. Since a greater force has to be applied, the possibility arises that the inner shaft may deflect or move under compression, as tension is applied to the pull wire. The thickness of the inner shaft then must increase to withstand the compressive forces. Larger forces will also tend to break the connection between the pull wire and sheath.
Another disadvantage of such an arrangement is that the diameter of the outer sheath has to be at least large enough, for the inner shaft, the stent and the inner ring to be accommodated in the sheath. Furthermore, the outer rings increase the thickness of the device as well, at least if one is clamped directly on the inner ring, and may distort the outside surface of the outer sheath. Since an application of such an implant catheter delivery system is for delivery of stents by advancing the catheter system into a bodily lumen, a thicker diameter reduces the range of possible treatment opportunities, especially in the narrow confines of a patient's vasculature, or may give rise to a risk of injury as the catheter system is advanced.